Secondary electron emissive electrode



Feb 25, 1941- v. K. zwoRYKxN Erm. 2,233,276

ISECONDARY ELECTRON EMISSIVE ELECTRODE Filed March 25, 1938 2.Sheets-Sheet 2 ELECTUDE Gttomeg Patented Feb. 25, 19411 l 2,233,276

SECONDARY ELECTRON EMISSIVE ELECTRODE Vladimir K. Zworykin,Philadelphia, Pa., and Humboldt W. Leverenz, Collingswood, and John E.Buedy, Merchantville, N. J., assignors to Radio Corporation of America,a corporation of Delaware Application March 25, 1938, Serial No. 197,994

' 'z claims. (ci. 25o- 174) Our invention relates to secondary electrondled as an ordinary metal for the preparation of emission devices, andmore specifically to a secelectrodes, and from which high secondaryenlisondary electron emission device in which the secsion may beobtained without any special activaondarily emissive electrode may beformed by a tion -Within a vacuum. Another object of our o compound oralloy of metallic elements. The invention is to form compounds or alloysof elealloy is further characterized bythe fact that nov ments which inthemselves do not show a high activation is required to increase thesecondarysecondary emissive property but which, in comelectronemissibility of the electrode surface. bination, show satisfactoryemission of secondary Secondary electron emissive devices have beenelectrons. A still further object of our invenknown to those skilled inthe art. These prior tion is to form an alloy or intermetallic com- 10devices have included secondary emissive surpound of two or moreelements which separately faces which require expensive and carefulprepahave low emissive characteristics, but which have ration. Thestability of these surfaces, partica ratio of primary to secondaryelectron emission ularly at high temperatures, has been very poor. ofthe order of 5 to 7 at 200 volts and higher at Furthermore, it has beennecessary to prepare higher voltages, and which are stable.

these surfaces in a vacuum, and it has been difli- Our invention' willbe described by referring to cult to activate such surfaces for the morecomthe accompanying drawings, in Which plicated mechanical structuressuch as electron Figure 1 is a chart illustrating the prior artmultipliers. method of forming a secondarily emissive elec- `-0 Thesecondary emissive surfaces of the prior trode; 20

art devices have generally included a silver or Figure 2 is a chartindicating the method of silver-plated electrode. After cleaning, theelecforming a secon-darily emissive electrode in actrode is mountedwithin a suitable envelope. cordance with our invention; The envelope isevacuated to 1 10-2 mm. of mer- Figure 3 is a graph showing the emissivepropcury and then heated and evacuated to a preserties of elements andtheir compounds made in sure of the order of 1 1C|5 mm. of mercury.accordance with our invention;

After evacuation, oxygen is introduced into the Figureiis anillustrationof asecondarily emisenvelope and the pressure maintained at approxsivedevice; and

n imetely 1 mm. cf mercury. The silver surface cf Figure 5 is atemperature-composition curve 0f 'M the electrode is oxidized byapplying a potential the elements gold and magnesium- 30 of the order of500 volts which effects a glow dis- Referring te Fig- 1,y if' Will beObserved that charge. After the oxidation has been completed, elevensuccessive steps are required to form a any residual oxygen is exhaustedand thereafter caesium-silver oxide secondary electron emissive caesiumis distilled into the envelope. The Surface. In comparison, yreferenceis made to a. suitable surface whichy is fixed by baking the formed ofthe elements Copper and magnesium. envelope at approximately 210 C.During the After the alloy has been formed, it is rolled into baking,excessive caesium is exhausted from the sheets from which electrodes ofthe desired size envelope, which is thereafter sealed at a pressure andshape may be stamped and formed. These 40 of approximately l l05 mm. ofmercury. While electrodes, after cleaning, are mounted within an 40 suchsurfaces show an electron multiplication envelope Sileh 94S ShOWn inFig- 4 The envelope factor of the order 0f 5 to 7 at 200 volts, the withthe electrodes mounted therein is heated for devices are, as pointed outabove, unstable and about 'one hour at 400 C. to 450 C. It is then arenot entirely suited to applications of high evacuated t0 approximately 1105 mm. of merpower with the resulting high electrode temperacury andsealed. Without activation or further 45 tures. preparation, we havefound that the alloy elec- There have been other attempts to producetrode will show a suitable ratio of primary to secondary emissivesurfaces Without employing secondary emission and, further, that thedevice alkali metals, but such attempts require that the will berelatively stable at ordinary operating emissive surfaces be formedwithin a vacuum temperatures. 50 and, as far as we have been able toascertain, Referring to Fig. 3,V it will be noted that the were not verystable at high temperatures. One element silver has .a secondaryemission ratio, of the primary purposes of our invention is toi. e., theratio of primary electrons striking the nd means for producing asecondary emissive element to the number of secondary electronselectrode which can be prepared in air, hanemitted'by such impact,of theorder of unity. 55

caesium combines with the silver cxide tc fcrm Fig. 2 in which,'by Wayof example, an alloy is 35 or broughtl together in an alloy, thecompoundor the alloy does not follow the ordinary rule of chemical mixtures. Thecharacteristics of the alloy depend not only on the proportions butalsoon the treatment which may result in many.

diiferent molecular arrangements. ySome oi|` the diiferent moleculararrangements of a compound or valloy'which results from this treatmenthave been clearly illustrated by means f electron defraction and X-rayanalysis. We therefore believe that the arrangement of the molecules ofthe compound of copper and magnesium readily lends itself to a surfacewhich liberates secondary electrons at relatively low primary electronvelocities. i By way of example, we have found that good secondaryemissive material may be made of alloys of magnesium with-copper, silveror gold, the percentage of magnesium to the base metal element beingequal to or less than the amount contained in a thermo-dynamicallystable intermetallic compound of the elements.

By the term thermo-dynamicallystable is meant stability of the electrodeunder baking and operating conditions term compound of elements in whichthe elements are represented in true atomic proportions; e. g., gold -Auand magnesium Mg form the following compounds; AuMg, AuMga and AuMga, asindicated by the maximum point or points in the temperature-compositiondiagram of Fig. 5.

By way of example, we have found that a fraction of one percent to theorder ot sixteen percent, by weight, of magnesium may be combined withcopper to form a suitable alloy. By way `of further examples, thefollowing proportions may be used: of the order of 84 parts copper to 16parts magnesium; of the order 80 parts silver to 20 parts magnesium; ofthe order of 70 parts gold to 30 parts magnesium; or 70 to 99.9

parts base metal to 30 to .1 parts magnesiumall by weight. Such an alloycan be readily rolled to the desired thickness and a stamping made ofthe desired shape to form suitable electrodes. 'I'hese electrodes, afterbeing introduced into a vacuum as described in connection with Fig.` 2,

Y will give from 4 to 6 and higher secondary electron ratios at 200volts and upwards at higher potentials,v in accordance with the curve ofFig. 3. We have found that such electrodes have surfaces which, whenfirst subjected to electron bombardment, show a decrease in secondaryemission. 'I'he surface eventually stabilizes with a comparatively highgain and thereafter remains stable at temperaturesup to 600 C.

'I'he foregoing example is not to'be considered a limitation. We havemade other secondary emissive electrodes of suitable emission ratio byalloying magnesium and gold, and magnesium and silver, in amounts equalto or less than the thermo-dynamically stable intermetallic compounds.It should also be understood that our use of the word "alloys includesintermetalllc compounds of the elements. 'I'he use of two elein avacuum. By they is meant a union or mixture y tures approaching 600 doesnot exclude the useofa plurality of elements; for example,silver-copper-magnesium alloy. 'I'he iinal criterion of a suitableelectrode..

is that it should be stable at highoperating temperatures and preferablyrequire no activation; therefore, we prefer to use alloysy of elementswhich are thermo-dynamically stable at operat-l ing temperatures abovethe envelope baking temperature and which should have suitably highratios of electron emission. vDetailed descriptions of secondaryelectron emitters or multipliers will be found in the copendingapplication SerialNo. 92,566, filed 'July 25, 1936, by Vladimir K.Zworykin and Louis Malter.

We claim: i

1. A secondary electron emissive electrode including in combination abase metal element and magnesium united in a ratio of at/least two partsbase metal to one part magnesium by weight and forming an alloythermodynamically stable at operating temperatures approaching 600 C.during electron bombardment, and having a ratio of secondary electronemission to primary electron applied during bombardment oi' at leastfour. l

2. A secondary electron emissive electrode formed bya compound of theelements copper and ma nesium in the ratio of the order of 84 parts cperand 16 parts magnesium by weight,

, and having a molecular structure which exhibits formed by a compoundof the elements silver' and magnesium inthe ratio of the order of 8 0.l

parts silver and 20 parts magnesium by weight,

and having a molecular structure which exhibits l thermodynamicstability at, operating tempera- C. and a capacity to emit secondaryelectrons during bombardment by primary electrons.

4. A secondary electron emissive electrode formed by a compound of theelements gold and magnesium in the ratio of the order of 70 parts goldand 30 parts magnesium by weight.

5. The method oi makinga secondary emissive electrode from a base metaland magnesium which comprises alloying said base metal and saidmagnesium, forming an electrode of said alloy, cleaning said electrode,placing said electrode within an envelope, heating said envelope toapproximately 450"v C., and evacuating said envelope.

6. The method of making a secondary emissive electrode from base metaland magnesium which comprises alloying said base metal and saidmagnesium, forming an electrode of said alloy, cleaning said electrode,placing said electrode within an envelope, heating said envelope attemperatures from substantially 400 to 450 coL C. and until saidsecondary emission becomes sta-

